An opening or vent is commonly provided in a tank (such as a reactor, a processing device, a storage tank, a gas holder or other vessel) for the discharge, or the release, of an undesirably large pressure differential relative to the adjacent environment. To regulate use or operation of the tank, avoid material loss from the tank, prevent damage to the tank, and/or minimize or even prevent environmental contamination with material from the tank, such a vent is commonly provided with a vent valve that opens at a preset pressure differential.
While various vent valve structures are known, one particularly effective structure utilizes a combination of pallet and diaphragm. The pallet in normal valve operation extends across and closes the valve mouth while the associated diaphragm seats about and seals the valve mouth. Various means are known for regulating the opening or set pressure of such a vent valve where the pallet and diaphragm combination separates from the valve mouth. For example, one particularly effective pressure regulating means comprises a weight that is loaded onto the upper outside surface of the pallet. Thus, in such an arrangement, the weight-loaded diaphragm separates from its associated seat at a preset tank pressure (which can be superatmospheric or subatmospheric), thereby to achieve vent valve opening.
However, one problem with such a pallet and diaphragm-type vent valve is that a small volume of fugitive vapor emissions (typically parts per million) may occur between the diaphragm and its seat before the set diaphragm opening pressure of the vent valve is reached.
The present invention overcomes this problem by providing a liquid seal that is cooperatively associated with a pallet and diaphragm-type vent valve. The liquid seal functions to virtually eliminate the escape of all fugitive vapor emissions prior to the opening of the pallet and diaphragm combination at a set opening pressure.